Staphylococcus aureus is an opportunistic pathogen that is associated with a range of serious pathogenic conditions in humans and animals. The emergence of virulent, community-acquired S. aureus strains that are resistant to many antibiotics has become a significant public health problem. Many virulence determinants in S. aureus are carried on mobile genetic elements, including S. aureus pathogenicity islands (SaPIs) that carry genes encoding virulence factors such as superantigen toxins. SaPIs are normally stably integrated into the host genome, but become mobilized by specific helper phages, such as 80a or fNM1, resulting in the packaging of the SaPI genome into phage-like transducing particles that are made from helper-encoded structural proteins. The SaPIs have evolved the ability to sense the presence of a lytic phage, exploit phage functions and interfere with phage multiplication. This interference includes altered dependency on phage functions, specific selection of SaPI DNA for packaging, and the formation of capsids that are too small to package phage genomes. The resulting particles can transfer virulence factors to other cells, even across genera, at high frequency. The overall aim of the current project is to understand the structural basis for SaPI mobilization, helper-SaPI specificity, and the factors involved in their spread and establishment. Such processes are important factors in S. aureus evolution and pathogenicity. Specifically, we aim to: (1) Define the roles of the 80a scaffolding protein in capsid assembly; (2) Determine the mechanism of SaPI-induced capsid size redirection; (3) Establish the role of gp44 in DNA packaging and stability; and (4) Understand the process of SaPI derepression by helper phage dUTPases. These aims focus on different aspects of the mobilization process and will be studied by a combination of genetic, biochemical and structural methods. Elucidation of these mechanisms will have significant implications for understanding the role of phages, SaPIs and other mobile genetic elements in the spread and establishment of virulence determinants in S. aureus. In addition, this research will aid our understanding of macromolecular assembly processes and capsid size determination in general.